1. Field of the Invention
The present invention relates to an image forming apparatus which forms a color image by superposing images of a plurality of colors formed by a plurality of print units.
2. Description of Prior Art
In a color image forming apparatus of tandem structure, an image of four colors is formed at the same time in a scan. In the color image forming apparatus, four printer units including photoconductors for the four print colors are aligned, and images of the four colors are supplied to exposure positions for the colors, by delaying in correspondence to the distances between the photoconductors. In such an apparatus of tandem structure, print positions of the color images are liable to be shifted, due to delay of image data through memories, shifts of exposure position on the four photoconductors and the like. Then, a color shift may arise in an image to cause image distortion such as bow distortion.
Then, in order to produce an image of high quality, it is desirable to correct image distortion. It is proposed to detect image distortion and to change line address for each color in the subscan direction in correspondence to an address in the main scan direction based on the detected image distortion. However, in this case, correction can be performed in the unit of address, but not in the unit of a smaller size. Further, because the correction in the main scan direction is not performed, management of correction data is complicated for performing delicate correction.
Further, in the above proposal, if the detected image distortion exceeds the maximum image distortion (or line number), image forming is inhibited. Then, printing may be inhibited frequently.